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We can now identify where list comprehensions are used. If you noticed, human is a string, not a list. This is the power of list comprehension. It can identify when it receives a string or a tuple and work on it like a list. You can do that using loops. However, not every loop can be rewritten as list comprehension. Finding squares using list comprehensions: Here, square brackets signifies that the output is a list. N.2 is the expression executed for each element and for n in numbers is used to iterate over.
This question already has an answer here:
- if/else in a list comprehension? 7 answers
I have a list
l
:For numbers above 45 inclusive, I would like to add 1; and for numbers less than it, 5.
I tried
But it gives me a syntax error. How can I achieve an
Keith Pinsonif
– else
like this in a list comprehension?4,84955 gold badges4242 silver badges8888 bronze badges
user225312user22531267.6k5959 gold badges148148 silver badges174174 bronze badges
marked as duplicate by Dan D. pythonApr 4 '17 at 6:42
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
8 Answers
Do-something if
tscizzle<condition>
, else do-something else.2,38277 gold badges3232 silver badges6161 bronze badges
user225312user22531267.6k5959 gold badges148148 silver badges174174 bronze badges
The reason you're getting this error has to do with how the list comprehension is performed.
Keep in mind the following:
Is equivalent to:
Where the
expression
is in a slightly different format (think switching the subject and verb order in a sentence).Therefore, your code
[x+1 for x in l if x >= 45]
does this:However, this code
arboc7arboc7[x+1 if x >= 45 else x+5 for x in l]
does this (after rearranging the expression
):4,17022 gold badges2222 silver badges2929 bronze badges
And for a reward, here is the comment, I wrote to remember this the first time I did this error:
Python's conditional expression is
a if C else b
and can't be used as:The right form is:
Even though there is a valid form:
But that isn't the same as that is how you filter by
Dan D.Dan D.C
, but they can be combined:56k1010 gold badges8181 silver badges101101 bronze badges
You must put the expression at the beginning of the list comprehension, an if statement at the end filters elements!
AndiDogAndiDog51k1414 gold badges135135 silver badges187187 bronze badges
You can also put the conditional expression in brackets inside the list comprehension:
[false,true][condition] is the syntax
Stefan GruenwaldStefan Gruenwald
Like in
[a if condition1 else b for i in list1 if condition2]
, the two if
s with condition1
and condition2
doing two different things. The part (a if condition1 else b)
is from a lambda expression:while the other
condition2
is another lambda:Whole list comprehension can be regard as combination of
Xiaojun ChenXiaojun Chenmap
and filter
:46111 gold badge66 silver badges1717 bronze badges
I just had a similar problem, and found this question and the answers really useful. Here's the part I was confused about. I'm writing it explicitly because no one actually stated it simply in English:
The iteration goes at the end.
![List List](/uploads/1/2/5/2/125260873/952145468.jpg)
Normally, a loop goes
Everyone states the list comprehension part simply as the first answer did,
but that's actually not what you do in this case. (I was trying to do it that way)
In this case, it's more like this:
szeitlinszeitlin![List comprehension python two for loops List comprehension python two for loops](https://treyhunner.com/images/list-comprehension-condition.gif)
1,77111 gold badge1414 silver badges1414 bronze badges
You could move the conditional to:
But it's starting to look a little ugly, so you might be better off using a normal loop. Note that I used
JeetJeetv
instead of l
for the list variable to reduce confusion with the number 1 (I think l
and O
should be avoided as variable names under any circumstances, even in quick-and-dirty example code).27.7k55 gold badges4040 silver badges4747 bronze badges